As corn margins squeeze production even tighter, cutting costs is important. But don't count on reducing seeding rates or plant populations to slash inputs. You'll likely be rudely rewarded with reduced yields.

In fact, with the enhanced performance traits seen in multiple-stacked 21st-century corn hybrids, using proper seeding rates could be the best management practice growers can use.

Sure, costly kernels can easily top $100+/acre. But Corn Belt agronomists see proper plant populations as essential in obtaining the yields needed to produce profits.

Jeff Coulter, University of Minnesota agronomist, says the optimum plant population for the North Star state is around 33,000 plants/acre for today's hybrids. “The optimum plant population will be slightly lower in low-yielding areas and maybe a little higher in high-yielding areas, but will probably not vary more than 3,000 plants/acre in a given field,” he says.

Adds Bob Nielsen, Purdue University agronomist, “Most of our growers (in Indiana) probably need to be targeting no fewer than 30,000 plants at harvest. That would require seeding rates of 33,000. And that number is higher than some of our farmers are currently planting.”

IN THE EASTERN Corn Belt, “growers may need to consider seeding rates 10-15% higher than the desired harvest population when planting early using no-till on poorly drained soils with heavy residues,” says Peter Thomison, Ohio State University agronomist.

“Seeding rates will certainly vary with different hybrids,” he says. “Some companies recommend final stands of 32,000 and higher in exceptionally high-yield production environments. That means growers need seeding rates higher than that.

“You're looking at between 35,000 and 36,000 seeds/acre, depending on the seeds' germinability and potential for mortality after planting,” he says. Since stalk rots and lodging often increase with higher seeding rates, growers should follow seed company recommendations to adjust plant population for specific hybrids.

Typically, he says on soils that average 120 bu./acre or less, final stands of 20,000-22,000 plants/acre are adequate for optimal yields. On soils that average about 150 bu./acre, a final stand of 28,000-30,000 plants/acre may be needed to optimize yields.

For sites with high yield potential with high soil-fertility levels and water-holding capacity with long-term average yields of 175 bu./acre or more, final stands of 32,000-33,000 plants/acre may be required to maximize yields.

Calculating seeding rates, Barry Ward, Ohio State University Extension economist, says his projected 2009 corn budget calls for an average seed cost of $200/bag and a per-acre cost of about $85 for a 34,000-seed planting. But current seed costs can equate to over $100/acre. (With some hybrids at $250-300/bag, and each bag containing about 80,000 seeds, that translates to each bag furnishing seed for just over two acres at the 33,000-34,000 seeding rate.)

But the higher rates are needed in many fields. Thomison says modern corn hybrids respond better to higher seeding rates, adding that seeding rates have increased consistently since the early 1970s. In Ohio, for example, seeding rates were in the 18,000/acre range from 1971 to 1975, jumped to 21,000 by 1986, 23,000 by 1996 and just under 28,000 in 2008. Look for it to be higher this year and beyond.

“Growers should remember that if the recommended plant population for a corn hybrid is 28,000 plants/acre, that refers to final plant stand or number of plants per acre at harvest, not seeding rate,” he says. “The number of plants per acre at harvest is always less than the number of seeds planted.”

COULTER SAYS POTENTIAL rainfall during the season, soil type and hybrid should be considered when choosing a seeding rate.

The planting date, tillage practices, pest problems, chemical injury, planter performance and seed quality can affect final corn populations. To compensate for these losses, a corn grower usually needs to plant more seed than the desired population at harvest.

Thomison uses this formula for determining a seeding rate: Seeding rate = desired plant population per acre at harvest ÷ (seed germination x expected survival). Seed germination is the percent germination shown on the seed tag.

“Most seed corn has a germination rate of 95% or higher,” he says, pointing out that survival rates for corn are often in the range of 85-95% “but can vary considerably depending on planting conditions and other environmental factors.

“If planting early or if there are conditions that will likely create stress for corn during emergence, consider seeding rates 10-15% higher than the desired harvest population.”

For example, take a grower who wants to achieve a final stand of 28,000 plants/acre. The seed tag indicates a germination rate of 95% and the grower expects that 90% of the germinable seed will survive until harvest. Based on the formula above, dividing the desired plant population at harvest, 28,000 plants/acre, by 0.95 × 0.90 (0.855) converts to a seeding rate of nearly 33,000 seeds/acre.

Nielsen says yields could easily suffer if the seeding rate isn't 10-15% above the desired plant population. “If the objective is that 30,000 final plant population, I would encourage the correct seeding rate,” he says.

“There are some who will tell you to drop even more than about 33,000. I wouldn't agree with that. If you're dropping 36,000-37,000, you can probably cut back and not risk losing yields,” he says.

Nielsen doesn't believe new multiple-trait hybrids “are miraculous in yield potential.

“But today's hybrids, biotech or not, are much better than they were 20 years ago,” he says. “They (public and private corn breeders and seed companies) have done a wonderful job of improving overall stress tolerance.

“When growers run into drought and other stressful weather, they are getting through those with amazingly good yields,” says Nielsen. “I attribute that to broader improvements, whether it's a three-trait system, two-trait system or one- trait system.”

Coulter notes that optimum plant population for corn in Minnesota varies little with planting date or row width, but early maturing hybrids may require a higher plant population than full-season hybrids. Yield increases resulting from higher plant populations are primarily the result of increased light interception during grain-fill by the crop canopy.